The Many-Worlds Interpretation (MWI) is an approach to quantum mechanics according to which, in addition to the world we are aware of directly, there are many other similar worlds which exist in parallel at the same space and time. The existence of the other worlds makes it possible to remove randomness and action at a distance from quantum theory and thus from all physics.

... every time a quantum experiment with different outcomes with non-zero probability is performed, all outcomes are obtained, each in a different world, even if we are aware only of the world with the outcome we have seen.

... Firstly, assuming that there are many worlds like this that exist at the same space & time, it should be acknowledged that these worlds cannot be comprised of the same matter, for if one (the same) set of matter exists at one (the same) set of space & time, then we only have one world, not many. That is, the MWI requires that different matter/bodies exist at the same space & time. But, how can more than one body exist at the same space & time? That is illogical. (Problem 1)

Secondly, ignoring the first problem, let us focus upon the claim that "The existence of the other worlds makes it possible to remove randomness and action at a distance from quantum theory and thus from all physics.". No it does not, is my retort. Why? Because if what distinguishes one world from another is the distinct/separate bodies that already exist in each of these worlds, then whichever 'experiment with different outcomes with non-zero probability is performed' the outcome in each of these worlds is still indeterminable. Why? Because one body cannot jump into another world where a pre-existing body of an exact likeness already exists. How do I know this? Because I am yet to see more than one of anything... and if it were the case that mirroring-bodies could jump from one universe to the next, then we would expect to see numerous variants of some bodies such as, say, Stonehenge, all co-existing together. In other words, almost everything would be an observed blur - the consequence of it existing simultaneously with variances of its potential.

... Bodies a; b; c; d; e, etc, already exist in this world. The MWI would have it that different like-bodies of a; b; c, etc, exist in another world. So, whatever happens to those bodies will happen to them in their world. It won't happen to them in this world. Therefore, the MWI does not eradicate randomness from quantum theory. Further, it cannot explain nor eradicate the problem of 'action at a distance', for the same problem arises: even if there are many Stonehenges - what happens to 'it' will happen to it in its own world... since it cannot transcend that world into another. (2nd problem)

Lastly, for now, the MWI is physically dependent. That is, the likelihood of an outcome is assessed from physical potential. However, we all know that the likelihood of events isn't contingent upon physical potentials. I know, for instance, given the evolution of my own life/mind, that the likelihood of me becoming a materialist tomorrow, is zero. I have no doubt about that, given that I've already been there and seen the flaws thereof (not to mention everything else I've 'seen'). Likewise, you all may be sure of some thing or other. Further, for example, though the physical potential exists, the likelihood of tomorrow's papers headlining The Pope as a murderous gay atheist, seems bleak, to say the least. Therefore, are these many worlds constrained by what is physically possible, or by what is sensibly possible? That is, do mental/emotive concerns dictate what worlds are possible, or simply physical potentials? On the face of it, it would seem that the MWI doesn't have any recourse towards mental potential/agency. (3rd problem).

I started this thread because two people informed me that the MWI is not irrational and that any assault upon it was bound to be a consequence of incredulity, on my part. In other words, I started this thread to try and prove why reason can undermine the MWI. I expect to think of further problems with it as the discussion evolves.

... Any serious input is welcome. Btw, I'm supposed to beextremely busy with something else until next Tuesday evening, so pardon me if my responses seem a bit light up until that point. However, my mind has become focused upon this issue and I felt compelled to put it out there. Thankyou.

jamest wrote:... Firstly, assuming that there are many worlds like this that exist at the same space & time, it should be acknowledged that these worlds cannot be comprised of the same matter, for if one (the same) set of matter exists at one (the same) set of space & time, then we only have one world, not many. That is, the MWI requires that different matter/bodies exist at the same space & time. But, how can more than one body exist at the same space & time? That is illogical. (Problem 1)

MWI does not propose that that "more than one body exists at the same time" in any fundamental way. To put it simply, it proposes that the basis for "reality as we know it" is the quantum wave function, and that this has real existence. Parts of the quantum wave function become "decoherent" from other parts, meaning that they cease to interact in any way worth talking about. This gives the appearance of separate worlds, with separate objects. Suppose a ball exists in my world, and I can interact with it. Another ball exists in someone else's world, and I can't interact with that. The two balls don't interact with each other - even though that might be in the same space. The underlying explanation of this isn't that "two bodies occupy the same space". It is that a single wave function occupies the same space, and parts of it don't interact with other parts - and yes, the two parts can overlap. However, the idea that waves can overlap is hardly radical.

A (very crude and open to misuse) analogy might be radio transmissions. Someone might ask how radio transmissions of two separate radio shows can pass through the same space at the same time. Doesn't this require two things to occupy the same space? This is dealt with when we see that nothing really passes through the space except radio waves - and a single description can be made of the radio waves passing through the system. Likewise, you could step back from the many-worlds of MWI and describe everything in terms of just "one world" with the wave function - and parts of that wave function ceasing to interact with other parts explain the appearance of separate worlds. The different worlds in MWI aren't really separate in any ontologically "profound" way but merely by lack of interaction and limited human perception due to our status as observers embedded in the system.

In a way, the concept of matter as we know it is no longer fundamental in MWI anyway. It makes little sense to object to a ball in one world being "the same matter" as the ball in another world. Looked at closely enough it is all just waves anyway.

Even thinking of this in terms of "bodies" existing in different worlds suggests a problem to me, so I have doubts about whether it is worthwhile getting into the other questions right now. I will say that MWI makes no claim that the results of quantum mechanics experiments should be predictable by humans.

If I ever start making posts like "On the banning and partial banning of words!" then I view my life as less than worthless and I hope that my friends here would have a collection to pay for ninjas to be sent to my home to kill me*. (*=humanely)

Therefore, are these many worlds constrained by what is physically possible, or by what is sensibly possible?

The former. In some branch of an MWI multiverse, somebody is in the Guinness Book of Records for rolling 100,000 consecutive sixes without cheating....

Is MWI sensible? Depends what non-sensible things you are trying to get rid of.

You seem to have overlooked the first two problems, focusing instead upon addressing the 3rd problem, lightly. When you send me a cheque for £10,000, I will take heed of your response to the 3rd problem.

Therefore, are these many worlds constrained by what is physically possible, or by what is sensibly possible?

The former. In some branch of an MWI multiverse, somebody is in the Guinness Book of Records for rolling 100,000 consecutive sixes without cheating....

Is MWI sensible? Depends what non-sensible things you are trying to get rid of.

You seem to have overlooked the first two problems, focusing instead upon addressing the 3rd problem, lightly. When you send me a cheque for £10,000, I will take heed of your response to the 3rd problem.

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jamest wrote:... Firstly, assuming that there are many worlds like this that exist at the same space & time, it should be acknowledged that these worlds cannot be comprised of the same matter, for if one (the same) set of matter exists at one (the same) set of space & time, then we only have one world, not many. That is, the MWI requires that different matter/bodies exist at the same space & time. But, how can more than one body exist at the same space & time? That is illogical. (Problem 1)

MWI does not propose that that "more than one body exists at the same time" in any fundamental way. To put it simply, it proposes that the basis for "reality as we know it" is the quantum wave function, and that this has real existence. Parts of the quantum wave function become "decoherent" from other parts, meaning that they cease to interact in any way worth talking about. This gives the appearance of separate worlds, with separate objects. Suppose a ball exists in my world, and I can interact with it. Another ball exists in someone else's world, and I can't interact with that. The two balls don't interact with each other - even though that might be in the same space. The underlying explanation of this isn't that "two bodies occupy the same space". It is that a single wave function occupies the same space, and parts of it don't interact with other parts - and yes, the two parts can overlap. However, the idea that waves can overlap is hardly radical.

A (very crude and open to misuse) analogy might be radio transmissions. Someone might ask how radio transmissions of two separate radio shows can pass through the same space at the same time. Doesn't this require two things to occupy the same space? This is dealt with when we see that nothing really passes through the space except radio waves - and a single description can be made of the radio waves passing through the system. Likewise, you could step back from the many-worlds of MWI and describe everything in terms of just "one world" with the wave function - and parts of that wave function ceasing to interact with other parts explain the appearance of separate worlds. The different worlds in MWI aren't really separate in any ontologically "profound" way but merely by lack of interaction and limited human perception due to our status as observers embedded in the system.

In a way, the concept of matter as we know it is no longer fundamental in MWI anyway. It makes little sense to object to a ball in one world being "the same matter" as the ball in another world. Looked at closely enough it is all just waves anyway.

Even thinking of this in terms of "bodies" existing in different worlds suggests a problem to me, so I have doubts about whether it is worthwhile getting into the other questions right now. I will say that MWI makes no claim that the results of quantum mechanics experiments should be predictable by humans.

Okay, thanks for the explanation. It appears that I've taken this 'many worlds' conception too literally. Anyway, if there are problems with the MWI I need to rethink them.

jamest wrote:Okay, thanks for the explanation. It appears that I've taken this 'many worlds' conception too literally. Anyway, if there are problems with the MWI I need to rethink them.

It depends what you mean by "too literally". What is important is that the "splitting" doesn't happen in any kind of (I don't even know what vocabulary I should use here) "ontologically profound" way. However, the theory does propose that these worlds do exist as far as we are concerned. If MWI is true, there will indeed be worlds in which the Nazis won World War II and a statue of Adolf Hitler is standing in London: the proportion of such worlds might be large or small depending on how lucky or not you think you Britain was during World War II. However, the only thing that makes these "other worlds" is the lack of interaction. To some being able to take a "Godseye" view of this, there would just be the wave function. This is the sense in which MWI is supposed to get rid of randomness. Randomness is still apparent to the inhabitants of a single world, but from the "Godseye" perspective, there is just the wave function, developing deterministically over time. To you, the randomness comes from having multiple futures at any time and continually being split as an observer.

Incidentally, a test for MWI was proposed, but I don't advise doing it.

If I ever start making posts like "On the banning and partial banning of words!" then I view my life as less than worthless and I hope that my friends here would have a collection to pay for ninjas to be sent to my home to kill me*. (*=humanely)

jamest wrote:Okay, thanks for the explanation. It appears that I've taken this 'many worlds' conception too literally. Anyway, if there are problems with the MWI I need to rethink them.

It depends what you mean by "too literally". What is important is that the "splitting" doesn't happen in any kind of (I don't even know what vocabulary I should use here) "ontologically profound" way. However, the theory does propose that these worlds do exist as far as we are concerned. If MWI is true, there will indeed be worlds in which the Nazis won World War II and a statue of Adolf Hitler is standing in London: the proportion of such worlds might be large or small depending on how lucky or not you think you Britain was during World War II. However, the only thing that makes these "other worlds" is the lack of interaction. To some being able to take a "Godseye" view of this, there would just be the wave function. This is the sense in which MWI is supposed to get rid of randomness. Randomness is still apparent to the inhabitants of a single world, but from the "Godseye" perspective, there is just the wave function, developing deterministically over time. To you, the randomness comes from having multiple futures at any time and continually being split as an observer.

Incidentally, a test for MWI was proposed, but I don't advise doing it.

You cannot arouse my curiousity like that and say nothing. What's the test?!

Paul Almond wrote:Incidentally, a test for MWI was proposed, but I don't advise doing it.

You cannot arouse my curiousity like that and say nothing. What's the test?!

The idea is known as "quantum suicide". You set up an experiment in which the following occurs automatically.

Some quantum event occurs with two possible results - heads or tails.If tails occurs, you are immediately killed by some automatic mechanism, before you had chance to realize. For example, a gun may be set up to shoot you in the head.

You set the apparatus up so that it will do this a large number of times - lets say 100 times.

If you are alive after going through many repetitions of this, the idea is that if MWI is false, it is very unlikely that you should be alive. On the other hand, if MWI is true there is always going to be a future in which you survive. The idea here, and it is obviously somewhat controversial, is that you never observe the futures in which you are dead, so they can be left out of your statistics: from your perspective, if MWI is true, just before you go through all this, you are supposed to have nothing to fear as you should fully expect to survive and, at the end of this, your continued existence will effectively have shown you that MWI is true.

One thing I should point out is that the idea that finding yourself alive is evidence that MWI is true is actually based on a Bayesian calculation. The idea is that, beforehand, you can calculate the probability of surviving if MWI is false (very low) and the probability of survving if MWI is true (supposedly 1 if you buy into this). No matter how unlikely you think MWI is, provided you give it some non-zero chance of being true, if you survive a long enough sequence like this, the idea is that the usual Bayesian calculation will say that probability of MWI being true is effectively increased towards 1 as much as you want.

Of course, there are objections to this, and they will tend to be based on one or more of the following:

1. MWI is false - if MWI is false, none of this is relevant.

2. If MWI is true, it doesn't follow that you can just rule out the branches where you die - ruling them out is relying on a particular view of human continuity. You might think that that you have a 50% chance of dying each time even if MWI is true, and if you do you will reject this.

3. Some people may raise issues with the quickness which your death results, thinking that this may be an issue.

4. Even if MWI is true, and even if you buy into the "you should expect to survive if MWI is true" idea, surviving doesn't actually tell you anything. I think that this is the case, and that the experiment tells you nothing - no matter what you assume about human continuity and whether the branches where you die are "futures" for you. Let me put it this way - if I were forced to go through this, and survived 1,000,000 heads/tails events, at the end of it, whatever probability I assigned to MWI being true would not have changed at all. This may seem counter-intuitive. I think some people would think, "This future exists in an MWI reality, but the chance of it existing in an MWI reality is so remote that I must be in an MWI reality." I disagree. The idea that this whole business tells you anything is, in my view, a misuse (accidentally) of the Bayesian method. I do actually have a proper argument to show this, but it is an exercise in "statistical ontology" and is a bit involved. It would be best for everyone just to take my word for it, though nobody will as this is a site for skeptics.

In any event, I am not suggesting that anyone should do this.

If I ever start making posts like "On the banning and partial banning of words!" then I view my life as less than worthless and I hope that my friends here would have a collection to pay for ninjas to be sent to my home to kill me*. (*=humanely)

Although it is called a 'many worlds' hypothesis, it sounds like the hypothesis being discussed is in fact an 'every world hypothesis', in the sense that it seems to say that every possible world (possibly constrained by some set of initial conditions) exists.

I find every-world hypotheses, whether in the quantum-mechanics based form or any other, somewhat morally disheartening. If whenever faced with a moral decision I believe that, regardless of what choice I make, there will always be just as many worlds in which I make the immoral decision, it seems to make it pointless to weigh up ethical choices. In short, there is nothing I can do to make the totality of all worlds any better.

Fortunately, the hypothesis seems destined to remain a speculative hypothesis for the foreseeable future, so we can rationally choose to believe it or not on other grounds, such as aesthetics.

Paul Almond wrote:To put it simply, it proposes that the basis for "reality as we know it" is the quantum wave function, and that this has real existence.

Functions are abstract mathematical entities, so the quantum wave function cannot be the basis of physical reality, which must itself be a concrete physical entity (a quantum field?). It can at most represent that basis mathematically.

"Perception does not exhaust our contact with reality; we can think too." – Timothy Williamson

Once again, Teuton demonstrates that making shit up is superior to the operation of reality.

What a fucking numb statement. The quantum wavefunction is the closest approximation to reality we have. It may or may not be reality, but your categorical assertion with regard thereto once again exposes your fuckwittery. You really should shut the fuck up about things you know fuck all about, or go and join the Henry Morris fan club.

andrewk wrote:I find every-world hypotheses, whether in the quantum-mechanics based form or any other, somewhat morally disheartening. If whenever faced with a moral decision I believe that, regardless of what choice I make, there will always be just as many worlds in which I make the immoral decision, it seems to make it pointless to weigh up ethical choices. In short, there is nothing I can do to make the totality of all worlds any better.

But it doesn't actually say that - some things would occur in more worlds than others. This is because any "high level" outcome like this is going to be the result of a long sequence of quantum events. Some kinds of world would result from a big proportion of sequences of quantum events and would be "common". Others would only result from a small proportion of quantum events. Are there any worlds in which you turn into a serial killer? Well, it is hard to see how we can rule it out. We can imagine a small, random change to your brain which nudges it just a bit towards serial killing, and another one, and still another. However, how many such nudges do we need, and if we are allowed to have a lot of such nudges, and then compare the number of resulting worlds with the ones in which the same number of events occurs and you stay sane, it is probably a very small proportion. In MWI, it remains the case that some outcomes are more likely than others. The difference is that even the unlikely outcomes still happen - they just happen in fewer worlds. A good way of looking at this is asking yourself what the chance is, in the absence of MWI, that you will do immoral things. If you suddenly found out that MWI is true you should merely convert that to "a proportion of future worlds in which you do immoral things". Another point - this moral issue, if it is one, is still there if MWI is not true! It just has a purely probabilistic nature without the "many worlds" issue.

Paul Almond wrote:To put it simply, it proposes that the basis for "reality as we know it" is the quantum wave function, and that this has real existence.

Functions are abstract mathematical entities, so the quantum wave function cannot be the basis of physical reality, which must itself be a concrete physical entity (a quantum field?). It can at most represent that basis mathematically.

That sounds like a semantics issue to me, really - about whether we can use the word "function" or whether we are being lazy with semantics and ontology: what is being claimed by MWI is that "something" exists and that the "something" is described by the quantum wave function, and that that "something" can explain both quantum mechanics and the appearance of separate events to us. The term "quantum wave function" is used by both advocates and detractors of MWI without too much issue - and I think they all understand what they mean.

If I ever start making posts like "On the banning and partial banning of words!" then I view my life as less than worthless and I hope that my friends here would have a collection to pay for ninjas to be sent to my home to kill me*. (*=humanely)

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Paul Almond wrote:In any event, I am not suggesting that anyone should do this.

I might be persuaded to do the experiment with just a coin.

I don't see how any of what you said proves anything about MWI. Even within just this world, if I were to flip a coin countless times then eventually I'll a hit a prolongued run of heads or tails. Such possibilities are expected even within the realm of a singular reality. Apparently, it's even possible for a monkey to type the complete works of Shakespeare, even within just this world. Too furry or not too furry, that is not the question. Sorry, I appear to have stepped into the world where I've become mad.

The point you made about not being able to observe one's demise - because one is dead - seems daft to me. MWI is contingent upon statistics, not observation. I mean, after all, there must be many worlds in which I'm already dead, so that the existence of these worlds is not contingent upon my observation of them. Therefore, any analysis of MWI which seeks to prove the theory upon the basis of observation, seems like a dead duck, imo. Though perhaps the duck is alive elsewhere.

Last edited by jamest on Jun 12, 2011 1:00 am, edited 1 time in total.

Teuton wrote:Functions are abstract mathematical entities, so the quantum wave function cannot be the basis of physical reality, which must itself be a concrete physical entity (a quantum field?). It can at most represent that basis mathematically.

That sounds like a semantics issue to me, really …

No, that's a relevant ontological issue—really!

Paul Almond wrote:…- about whether we can use the word "function" or whether we are being lazy with semantics and ontology: what is being claimed by MWI is that "something" exists and that the "something" is described by the quantum wave function, and that that "something" can explain both quantum mechanics and the appearance of separate events to us.

You may say that the QWF describes, models, represents the physical basis of physical reality; but my point is that it is different from that basis, belonging to another ontological category. (Being a mathematical fictionalist, I don't believe in the real existence of abstract mathematical entities such as functions anyway.)

Paul Almond wrote:The term "quantum wave function" is used by both advocates and detractors of MWI without too much issue - and I think they all understand what they mean.

andrewk wrote:I find every-world hypotheses, whether in the quantum-mechanics based form or any other, somewhat morally disheartening. If whenever faced with a moral decision I believe that, regardless of what choice I make, there will always be just as many worlds in which I make the immoral decision, it seems to make it pointless to weigh up ethical choices. In short, there is nothing I can do to make the totality of all worlds any better.

But it doesn't actually say that - some things would occur in more worlds than others.

Yes my terminology was a bit loose there. I was aware of that as I wrote it, but couldn't be bothered doing the extra typing to state the thought more clearly - and got called out on it! What I was thinking - but not expressing very well - is that the hypothesis appears to suggest that nothing i can do can change the proportion of worlds in which I do the right thing.

Consider Meursault, the protagonist of Camus's l'Etranger, standing on the beach, pointing a gun at a man. As Robert Smith so chillingly puts it:

I could turn and walk away or I could fire the gunStaring at the sky, staring at the sunWhatever I choose it amounts to the sameAbsolutely nothing

Maybe there are only 1% of worlds in which Meursault shoots. But if he chooses not to shoot he hasn't changed those proportions. He has just selected which of the worlds he experiences. And by doing so he has implicitly made another version of him in another world kill the man and experience the consequences of doing so. In a sense, his choices are a zero sum game.

Another point - this moral issue, if it is one, is still there if MWI is not true

I don't follow this point. Can you expand on it? Are you referring to the conceptual problems surrounding the concept of free will?

Paul Almond wrote:To put it simply, it proposes that the basis for "reality as we know it" is the quantum wave function, and that this has real existence.

Functions are abstract mathematical entities, so the quantum wave function cannot be the basis of physical reality, which must itself be a concrete physical entity (a quantum field?). It can at most represent that basis mathematically.

According to MWI, the wave function is reality. In what way is a quantum field a "concrete physical entity"?